Many analog circuits have a long start-up time which is often controlled by an RC time constant. An example is shown in prior art circuit 10 of FIG. 1. Circuit 10, by way of example, has voltage reference source 2 ("V-REF. SOURCE") which is supplying, among other elements, load 4 ("LOAD") via resistor 6 of value R with filter capacitance 12 of value C. Load 4 is often an analog load.
In many applications, it is desirable that voltage Vo appearing on input 5 of load 4 from node 8 have very low noise. Resistor 6 and capacitance 12 act as a low pass filter to attenuate noise present on voltage Vi on output 3 from source 2. Resistor 6 is conveniently located within boundary 9 of integrated circuit (IC) 11 containing source 2 and load 4, but this is not essential. Resistance R can include the internal impedance of source 2. Capacitance 12 has input connection 7 coupled to node 8 and another lead coupled to reference 13, e.g., GND. Capacitance 12 is usually external to IC 11 because of its comparatively large size, but this is not essential.
It is often the case that the RC time constant T.sub.RC associated with circuit 10 can be relatively large, e.g., 10-1000 milliseconds. The time T.sub.S for the voltage Vo to stabilize after Vi is turned on is usually about T.sub.S =3.times.T.sub.RC. For many applications long stabilization times are undesirable. For example, the operation of a cellular phone can often require that it wake-up and go back to sleep many times a second. Having to wait 30-100 milliseconds for voltage Vo on node 8 and input 5 to stabilize is a great disadvantage. Such slow response time can adversely affect the cell-phone performance. Further, if the phone cannot respond quickly enough to a wake-up command, it may be necessary to leave it in a wake-state for longer periods of time, thus increasing power consumption and battery drain, and shortening the operating time between battery recharge. This is not desirable.
Thus, there continues to be a need for electronic apparatus with faster start-up characteristics, especially in connection with high impedance sources or when a low pass filter is included to reduce noise or both. It is desirable that faster start-up be achieved without great added circuit complexity and, if possible, without significantly increasing power consumption. Accordingly, a purpose of this invention is to provide electronic apparatus as recited in the claims which overcomes, in whole or part, these and other deficiencies or limitations of the prior art.